Ziegler-Natta catalysts are a mainstay for polyolefin manufacture. Much research has been done since their inception and there are many types of Ziegler-Natta catalysts. One useful Ziegler-Natta catalyst is disclosed in U.S. Pat. No. 4,464,518. It is made from the reaction product of a halogen-containing vanadium or titanium compound with a mixture of an organomagnesium compound and a silicon-containing compound. The catalyst and cocatalyst are described as being useful for ethylene polymerizations and ethylene copolymerizations with alpha-olefins such as propylene, 1-butene, 1-hexene or 1-octene. There is no indication that this catalyst would be effective for the copolymerization of ethylene with other types of olefins.
Silicon compounds have been used as donors with Ziegler-Natta catalysts. M. Harkonen, J. V. Seppala and T. Vaananen, Makromol. Chem. 192 (1991) 721 report that external donors markedly increase the sterospecificity and usually decrease the activity of Ziegler-Natta catalysts and that the generally accepted view of the role of the donor is a selective deactivation of active centers. Y. V. Kissin, J. Polym. Sci. Part A: Polym. Chem., 33 (1995) 227, reports a series of ethylene-hexene copolymerization experiments with varying amounts of diphenyldimethoxysilane. Increasing levels of silane decreased the 1-hexene incorporation and they concluded that the silanes poison different catalytic centers to different degrees.
There are many other instances of the use of low levels of silicon compounds as electron donors to modify a Ziegler-Natta catalyst. See, e.g., U.S. Pat. Nos. 6,559,250; 6,359,667; 6,362,124; 6,337,377; 5,595,827; and 4,900,706. The silicon compounds are often alkoxysilanes and can include vinylalkoxysilanes. The vinyl group is not required for the alkoxysilane to act as a catalyst donor. The silicon compounds are not used as comonomers and there is no indication that there could be any incorporation into the polymer chain.
U.S. Pat. No. 5,275,993 teaches a solid component for a Ziegler-Natta catalyst and the modification of the solid component with several components other than the essential titanium, magnesium and halogen components. Electron donors, silicon compounds, vinylsilane compounds and organoaluminum compounds are listed as possible modifiers. The vinylsilane is not used as a comonomer and there is no indication that there could be any incorporation into the polymer chain.
Copolymers of ethylene with vinylsilanes are known. U.S. Pat. Nos. 3,225,018 and 3,392,156 disclose free-radical copolymerizations under high pressure and temperature in the presence of a free-radical initiator. U.S. Pat. No. 3,225,018 teaches that the copolymerizations are generally conducted at pressures of 69 to 690 MPa and that the resultant copolymers can be crosslinked. U.S. Pat. No. 3,392,156 teaches reaction pressures of 103 to 310 MPa and that the copolymers have improved stress-crack resistance.
Another approach has been to graft vinylsilanes to polyolefins by heating them together in the presence of radical initiators such as peroxides. See, e.g., U.S. Pat. Nos. 6,465,107 and 4,902,460. These methods are difficult, have certain process hazards, can cause degradation of the polyolefin chain, and have limited flexibility to make a variety of polymers.
Incorporating silane functionality into polyolefins can impart crosslinkability for polyolefin products with improved strength and stiffness. It is also valuable for modifying flow properties or for bonding polyolefins to polysiloxanes, polyethers, polyurethanes, and other functionalized polymers. Despite the utility of copolymers of ethylene with vinylsilanes, it is apparently difficult to prepare these copolymers as evidenced by so few examples in the literature. The reported processes are limited and require very high pressure. Operating at such high pressure requires special equipment. There is a need for alternative processes to make ethylene-vinylsilane copolymers, preferably ones that work at relatively low pressures.